1. Field of Invention
This invention relates to a single-ended forward DC-to-DC converter with a new transformer secondary flyback reset scheme which allows its transformer to operate in both the first and third quadrants of B-H loop and extends its maximum duty cycle beyond 50% duty cycle.
2. Description of the Prior Art
The single-ended forward DC-to-DC converter is widely used in converting one DC voltage source to a different DC voltage supply for both small and medium power range due to its circuit simplicity and power conversion efficiency. However, the power transformer of this type of converter needs a transformer reset because the voltage applied to the power transformer is not a symmetrical AC waveform. The waveform actually consists of both a AC and a DC voltage components with the magnitude varies with both the input voltage and duty cycle. Without proper reset, the power transformer will saturate and result in a short circuit seen by the input voltage source.
There are known methods to reset the transformer including, for example, adding a reset winding, adding a capacitor in the primary across the main switch or across the forward rectifier diode in the secondary, to cause a resonant reset, and using a Zener clamp. In the first method, a reset winding with the number of turns equal to the primary winding is added and connected in series with a diode then across the input voltage source. The reset winding resets the transformer by providing a negative input voltage across its winding and recycles the magnetizing energy back to the input voltage source. The second example causes forward reset and dissipates the corresponding energy stored in the added capacitor through the main switch resulting in unwanted current stress on the switch, slowing down of the switch on/off operations, and higher switching losses. The alternative of connecting an added capacitor component across the rectifier diode increases failure rate, and adds cost and bulk to the converter package. In the third example, a Zener diode in series with a regular diode is connected across the transformer primary winding. The Zener clamp provides a negative voltage to the primary winding of the power transformer to reset its magnetic flux. The magnetizing energy in this case is dissipated by the Zener clamp.
There are major limitations when using one or more of these existing transformer reset schemes. First, the magnetic core of the power transformer only operates at the first quadrant of the flux density versus magnetizing force (B-H) loop and the third quadrant operation is not utilized. Second, the maximum duty cycle of the converter is essentially limited to 50%. Although the maximum duty cycle can be increased by either reducing the number of turn of the reset winding or increasing the Zener voltage, the blocking voltage requirement of both the primary main switching device and the secondary rectifier diodes will be proportionally increased. This is not recommended because the increase in voltage rating will increase conduction losses at both the main switching device and the secondary rectifier diode.
These limitations result in certain disadvantages including: a bigger magnetic core is needed compared to some other converter topologies (e.g., Push-pull converter); the RMS current is higher and causes larger power consumption; the current ripple is higher and causes excessive EMI; and the current stress on the main switch is undesirably large and causes higher switching losses.